It reduces the risk of letting people out of prison.

Regardless of the Supreme Court’s decision in Carpenter v. United States, using powerful technology to monitor criminal offenders will remain lawful. The Court may rule that police need a warrant to obtain suspects’ cell-phone records, but such a ruling won’t apply to parolees, probationers, or pretrial detainees. And that’s good, because monitoring such individuals electronically offers the most promising criminal-justice reform of the early 21st century.

Criminal suspects, like you and me, have strong constitutional protections where a “reasonable expectation of privacy” is implicated. But the Supreme Court has made clear that probationers and parolees have “severely diminished expectations of privacy.”

For instance, the Court once approved a California law providing that every prisoner eligible for parole must agree in writing to be subject to search or seizure by a parole officer at any time, day or night, with or without a search warrant or any justification whatsoever. A similar law for probationers also was upheld. For almost any other person this would be blatantly unconstitutional. But probationers and parolees are a special case.

Theoretically, these individuals are released to the community subject to strict limitations on their liberties, but big caseloads make close monitoring by probation and parole officers impossible. This creates a serious risk to the public. Two-thirds of all parolees, for instance, are arrested for a new crime within three years of discharge.

Fortunately, we now have the monitoring technology to dramatically improve the criminal-justice system. The Global Positioning System (GPS), familiar to millions of motorists and widely used by government and industry, is already being employed to track parolees, probationers, and pretrial detainees throughout the United States. It is used even more aggressively in Canada and Europe.

A radical expansion of GPS monitoring is the one criminal-justice reform that can both reduce incarceration and add to public security. Monitored offenders are much less likely to recidivate and end up back in prison. A Florida study supported by the U.S. Justice Department and involving over 5,000 medium- and high-risk offenders placed on electronic monitoring over a six-year period found that monitoring reduced recidivism by 31 percent. To get an idea of the significance of a one-in-three success rate, consider that in just one year (2015), over 340,000 probationers and parolees failed to comply with the requirements of release and were re-incarcerated. This is a significant portion of all people sent to prison that year.

Here’s how tracking works. Parolees, probationers, and arrestees released to bail are required to wear an ankle or wrist bracelet. The device enables probation and parole officers to monitor the subject in three different ways.

First, they can demarcate certain no-go areas — “geofenced exclusion zones” — so the device can alert authorities when the subject has physically entered a prohibited location. So, for example, a sexual-assault victim’s residence or workplace can be rendered electronically off-limits to the subject. If he enters the geofenced location, the GPS tracker will transmit to the monitors his location and the time of entry.

Every proposal to reduce incarceration that I’ve seen entails public danger.

Second, geofenced inclusion zones, such as a drug-treatment clinic or a place of employment, can be created to remind the subject of his obligations. The GPS device contains software that will audibly announce where and when the subject is supposed to go. The subject then will be able to check in on arrival, perhaps at a kiosk located within the facility.

Third, since the device tracks the movement of the subject along with time information, the authorities can geomap crimes in an area over a certain period and determine whether the subject was at or near a crime scene at the time of the offense. (In Carpenter, detectives similarly tracked the suspect’s proximity to cell-phone towers.)

Every proposal to reduce incarceration that I’ve seen entails public danger. If drug sentences are shortened, for example, or pretrial defendants are released through bail reform, there is an unavoidable risk of new crimes. For defendants with a long record of priors, that risk is even greater. Yet four of every five state prisoners are released to the community before completing their sentences. GPS monitoring is a natural fit for this situation. It is the only criminal-justice reform that is proven effective, can reduce incarceration, and will also keep the public safe.

— Barry Latzer is Professor Emeritus of Criminal Justice, John Jay College of Criminal Justice, CUNY. His most recent book is The Rise and Fall of Violent Crime in America (Encounter Books, 2016).He is working on a book on electronic monitoring of offenders.